Menopause explains female whales’ longevity

Very few female mammals go through menopause. For centuries, it was assumed to be unique to humans. But so far in the 21st century, up to five different species of whales have been discovered living well beyond the end of their fertility. All are odontocete cetaceans (who have teeth instead of baleen) and, like humans, spend their time in social groups that consist of several generations. The link found among these dozens of marine animals renders their connection to human beings all the more evident: they live longer to help the group, and spend the extra time taking care of their grandsons and granddaughters.

“The five species of odontocetes that evolved towards menopause live about 40 years longer than the expectation for members of the same species that do not go through menopause,” commented Samuel Ellis, researcher at the United Kingdom’s Exeter University, during an online conference. Ellis is the lead author of the study, which was published in the scientific journal Nature. The data aligns the whales with the life cycle of human women, for whom 42.5% of life takes place after their reproductive phase. Ellis pointed out that the end of the menstrual period shows up among different species, not all of whom share a common ancestor: killer whales, black killer whales (also referred to as false killer whales), pilot whales, narwhals and beluga whales.

In addition to outliving similarly sized females of other species, the females studied also outlive males from their own species. For example, female killer whales can live to 70 or 80 years of age, while males usually die at 40.

Why do they live longer? The mystery of menopause is that, from an evolutionary perspective, the longer the window of fertility, the better it is for the species. It seems nonsensical, and a waste of resources, to extend the life period with no chance of reproducing, and that this would be a selective disadvantage when compared to other species. In fact, of the more than 5,000 mammal species, only six (or seven, according to some studies — but we’ll get to that) have unpaired their longevity from ovarian production. The onset of the climacteric period could serve the same purpose, but by another route: the care not of children, but of grandchildren. Comparative analysis between different odontocetes revealed a key fact: for the females of species that have evolved into menopause, their post-fertile period overlaps with the lives of their grandchildren. Specifically, older killer whales, pilot whales and beluga whales live up to 36% longer when their daughters give birth to calves, as compared to similar species such as, respectively, the white-beaked dolphin, melon-headed dolphin and the Yangtze finless porpoise. “They thus have more time for intergenerational care,” said Ellis.

This amounts to the animal version of the grandmother hypothesis, or the idea that post-reproductive women are essential in human evolution. With offspring needing years of care, groups of humans have long been made up of several generations, and have a need for the transferal of cultural practices. With this in mind, individuals who lived for many years without being fertile were not a disadvantage — quite the contrary. Among killer whales, it has already been observed that calves live longer when they have their grandmother. A higher death rate among grandmother-less calves suggests a key familial role for post-menopause whales.

But the larger picture that has been noted among odontocetes is somewhat more complex. The same team of odontocetes researchers discovered a few years ago that the mortality rate of offspring increases dramatically when mothers are older. Specifically, when a mother and daughter have offspring at the same time, the former is 1.67 times more likely to die. In other words, in the competition for resources, the daughters of older mothers were at a disadvantage. These results point to a possible connection between the arrival of menopause and the end of the reproductive period based on the costs of reproduction.

“Females of these species have minimized competition for reproduction, lengthening their lifespan, but keeping the reproductive period shorter.”

Darren Croft, Exeter University research fellow

That’s the argument of Exeter University’s Darren Croft, a senior author of the study. “The second part of the story has to do with reproductive competition between generations,” he says. “What we see in the populations we’ve studied is that females of these species have minimized competition for reproduction, lengthening their lifespan, but keeping the reproductive period shorter. This is the same life pattern we observe among humans. It is quite striking that we can make this comparison between such different animals, but that have similar social structures and dynamic. It is very intriguing that we find this vital feature of human societies in the ocean, but not among other animals,” he says.

In the eyes of the researchers, many connections can be identified between human and animal menopause. As with humans, among killer whales (the species the team has most deeply researched), there are different forms of social organization. Regarding residents of the North Pacific coast, Croft provides a piece of information that fits into the grandmother hypothesis among humans: “One of the key benefits we have seen (in previous studies) whereby non-breeding females help the family group is by storing ecological knowledge of where and when to find food. This experience they gain throughout their lives is crucial when they have to deal with times of scarcity. And we see the same patterns in human hunter-gatherer societies in times of drought and in times of social conflict, when they turn to their elders.”

There is one other factor that human and odontocetes (but not other whales) share. In the majority of mammal species, offspring leave the group when they mature. Sometimes, both young females and males will set out on their own. On other occasions, only members of one gender will leave. “But for both to remain in the group (philopatry), both daughters and sons, is really rare among mammals,” Croft says. He compares this behavior to that of elephants, who also have complex social structures featuring grandmothers who have accumulated knowledge and care for offspring, but who do not go through menopause until the final stage of their lives. “One very striking difference between elephant and killer whale societies has to do with what happens to sons. Among killer whales, they stay with their mothers; among pachyderms, they leave,” he says. In fact, when it comes to the odontocetes, mothers continue to care for their older sons, something they don’t do with daughters after they reach reproductive age.

In light of this, it would seem that an evolutionary convergence has taken place in which similar selective pressures led to similar adaptive solutions. However, London School of Hygiene and Tropical Medicine researcher and reproductive expert Rebecca Sear recalls an example that does not fit into this schema: the recent discovery of a group of chimpanzees that go through menopause. “That is surprising, given that chimpanzees do not seem to provide much help to their grandchildren,” she writes in a comment also published in Nature.

Another possible bias among research that has thus far been conducted, Sear says, is that many studies on menopause in humans has tended to focus on finding evidence that grandmothers are helpful and, unsurprisingly, they have found such proof. “Contemporary grandmothers might help grandchildren because menopause evolved to create helpful grandmothers or because menopause means that older women have no choice but to spend time with grandchildren rather than children,” she says. To conclude, she offers an explanation that was put forth when it came to the chimpanzees: “There are many other hypotheses to explain menopause. One is that it is simply a sign of declining mortality, which has extended life expectancy in general, while reproductive life expectancy has remained the same.”

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